Fear-Potentiated Startle and Posttraumatic Stress Symptoms in Urban Police Officers

Transcription

1 Journal of Traumatic Stress, Vol. 16, No. 5, October 2003, pp ( C 2003) Fear-Potentiated Startle and Posttraumatic Stress Symptoms in Urban Police Officers Nnamdi Pole, 1,4 Thomas C. Neylan, 1,2 Suzanne R. Best, 1,2 Scott P. Orr, 2,3 and Charles R. Marmar 1,2 We studied the effects of increasing threat conditions on self-reported emotion, eyeblink electromyogram, and skin conductance responses to startling sounds in 55 police officers who endorsed a range of PTSD (posttraumatic stress disorder) symptoms. We found that contextual threat affected both physiologic and self-reported emotional responses. Greater PTSD symptom severity was related to greater physiologic responses under the low and medium but not under the high threat condition. The relationship between PTSD symptoms and physiologic responses was neither explained by self-reported emotional responses nor preexisting reported exaggerated startle symptoms. Our results emphasize the importance of contextual threat and suggest that laboratory measures of startle improve upon self-reported exaggerated startle alone in indexing PTSD symptom severity in urban police officers. KEY WORDS: PTSD; fear-potentiated acoustic startle; police officers; emotion. Patients suffering from posttraumatic stress disorder (PTSD) frequently complain about overreacting to sudden, loud noises (Southwick et al., 1995). In fact, the exaggerated startle symptom distinguishes PTSD from all other disorders in the Diagnostic and Statistical Manual of Mental Disorders (4th ed.; American Psychiatric Association, 1994). Like most psychiatric symptoms, this feature of PTSD is typically evaluated by patient selfreport. However, unlike most psychiatric symptoms, exaggerated startle can also be indexed with biological measures. Startling noises produce physiologic responses in at least two measurable domains, the striate muscle system (Berg & Balaban, 1999) and the autonomic nervous system (Gautier & Cook, 1997; Turpin & Siddle, 1983). Responses in the striate muscle system, which have usu- 1 Department of Psychiatry, University of California, San Francisco. 2 Department of Veterans Affairs Medical Center, San Francisco, California. 3 Department of Psychiatry, Harvard Medical School, Massachusetts General Hospital, Boston, Massachusetts. 4 To whom correspondence should be addressed at Department of Psychology, University of Michigan, 2260 East Hall, 525 East University, Ann Arbor, Michigan ; nnamdi@umich.edu. ally been assessed by measuring electromyogram (EMG) of the poststimulus eyeblink, are considered to be the primary measure of the human startle reflex. Responses in the autonomic nervous system, which have usually been assessed by measuring changes in the poststimulus skin conductance (SC) level and/or heart rate (HR), are not regarded as part of the startle reflex per se but rather are considered part of a general defensive response to aversive stimuli (Orr & Roth, 2000). Assessments of these physiologic domains could offer an alternative to self-report in the diagnosis of PTSD. This could be especially useful in populations where there may be motive to overreport symptoms (e.g., veterans seeking government benefits) or underreport symptoms (e.g., police officers who believe that their careers will be adversely affected by disclosing distress). Interestingly, evidence for an exaggerated physiologic response to loud sounds in PTSD has been mixed. Larger eyeblink EMG responses (EMGR) among PTSD patients have been reported by some investigators (Butler et al., 1990; Morgan, Grillon, Southwick, Davis, & Charney, 1996; Orr, Lasko, Shalev, & Pitman, 1995; Shalev, Peri, Orr, Bonne, & Pitman, 1997) but not by others /03/ /1 C 2003 International Society for Traumatic Stress Studies

2 472 Pole, Neylan, Best, Orr, and Marmar (Grillon, Morgan, Southwick, Davis, & Charney, 1996; Shalev, Orr, Peri, Schreiber, & Pitman, 1992). Larger HR responses (HRR) among individuals with PTSD have also been reported by most (Metzger et al., 1999; Orr, Solomon, Peri, Pitman, & Shalev, 1997; Paige, Reid, Allen, & Newton, 1990; Shalev et al., 1992) but not all (Shalev et al., 1997) investigators and larger SC responses (SCR) have been reported by some investigators (Shalev et al., 1992, 1997) but not by others (Orr et al., 1997). Several explanations have been proposed for these inconsistent results. One particularly compelling explanation is that group differences in startle responses may depend upon the level of fear induced by the experimental context (Orr et al., 1995). Fear has been found to amplify both EMGR (Lang, Bradley, & Cuthbert, 1990) and SCR (Vrana, 1995) to startling sounds. Threatening contexts have been found to increase the magnitude of the physiologic startle response in both animals (Brown, Kalish, & Farber, 1951) and normal humans (Grillon, Ameli, Woods, Merikangas, & Davis, 1991). Grillon, Morgan, and their colleagues have studied the effect of threat on EMGR to startling sounds in PTSD patients. In one such study examining the effects of being startled in the dark, they compared 19 veterans with PTSD, 13 veterans without PTSD, and 30 civilians without PTSD on their EMGR to light versus dark startle. Contrary to their hypothesis, they found that participants with PTSD had larger EMGR than did controls under both light and dark conditions (Grillon, Morgan, Davis, & Southwick, 1998a). Morgan, Grillon, Southwick, Davis, and Charney (1995) studied the effects of threatening 9 PTSD patients and 10 controls with electric shock. Again, contrary to their expectations, they found larger EMGR in the PTSD group, under both shock and no-shock conditions. These negative findings led to the hypothesis that knowledge of a future aversive event was enough to create a threatening context for PTSD patients. To test this possibility, Grillon, Morgan, Davis, & Southwick (1998b) assessed participants on two different days. On the 1st day, participants received startle without any threat of electric shock. On the 2nd day, participants received startle both with and without threat of shock. PTSD participants and controls did not respond differently on the 1st day but the PTSD group had larger EMGR in both conditions on the 2nd day, suggesting that the introduction of a threatening context, such as knowledge that shock will occur on the same day, was responsible for the exaggerated EMGR effect. No one has yet determined how (or if) threatening contexts influence autonomic startle responses in PTSD. In addition to enhancing the ability to accurately detect PTSD, understanding exaggerated startle may help us to understand the underlying pathology of PTSD. For example, if exaggerated startle is best observed under conditions of contextual threat then abnormalities in the processing of contextual information in the hippocampus and/or abnormalities in the management of stress in the noradrenergic system may be implicated (Morgan et al., 1995). Such a finding could also reflect general affective abnormalities such as a tendency to experience excessive negative, danger-related emotions (such as worry, fear, anxiety), and/or an inability to experience positive, safetyrelated emotions (e.g., calm, contentment). Although some studies have found expected differences in negative danger-related emotions (e.g., Grillon & Morgan, 1999), none have examined the role of positive safety-related emotions in startle even though recent work in emotion theory suggests that this might be an important area of study (Fredrickson, 1998). The Present Study In the present research, we examined PTSD symptom severity in relationship to physiologic and self-reported emotional responses to startling sounds under three levels of contextual threat. We studied these variables in a population in which these kinds of measures have not previously been studied, namely urban police officers. Urban police officers are of interest because they are routinely exposed to traumatic stressors and because they may have a tendency to underreport psychological distress. We hypothesized that (a) different levels of contextual threat would influence the magnitude of startle response; (b) PTSD symptom severity would be related to the magnitude of startle response; and (c) physiologic startle responses would predict PTSD symptom severity even after controlling for self-reported exaggerated startle symptoms and self-reported emotional responses to startling sounds. Method Participants Participants were a subsample of respondents to a larger survey on risk and resilience factors for PTSD in police officers (see Brunet et al., 2001, and Pole et al., 2001, for additional details on the composition and recruitment of the survey sample). Fifty-five officers from the San Francisco Bay Area were recruited on the basis of reporting either high or low PTSD symptoms for every level of exposure to duty-related critical incident stressors. All officers gave written informed consent and were reimbursed $40 for their participation in this study.

3 Startle, PTSD Symptoms, and Police 473 Diagnostic Interviews Officers completed a structured diagnostic interview within 6 months of their participation in the current experiment. The interview, which included both the Structured Interview for DSM-IV, Non-Patient Version (SCID; First, Gibbon, Spitzer, & Williams, 1996) and the Clinician- Administered PTSD Scale for DSM-IV (CAPS; Blake et al., 1997), was conducted by the third author, a PhDlevel clinical psychologist. The officers CAPS scores ranged from 0 to 57 (M = 11.3, SD = 16.1). Six (12.7%) officers met full CAPS criteria for current PTSD. All but one of these also met current criteria for one or more of the following: major depression, dysthymic disorder, alcohol abuse, cannabis abuse, social phobia, specific phobia, and/or generalized anxiety disorder. Among those who did not meet full criteria for PTSD, one met criteria for specific phobia, one met criteria for alcohol dependence and cannabis abuse, and seven met criteria for alcohol abuse. Laboratory Procedure Psychophysiologic data were collected by the first author, a PhD-level clinical psychologist, who was kept blind as to the diagnostic status of the participants. Officers reported that they consumed the following within 24 hr prior to startle testing: caffeinated beverages (M = 1.2, SD = 1.0), cigarettes (M = 0.15, SD = 1.09), and alcoholic beverages (M = 0.18, SD = 0.72). Three officers reported using medications that might alter autonomic or emotional responding (atenolol and verapamil). All analyses were run with and without these officers but none of the statistical conclusions were altered by their exclusion. Participants wore headphones and sat upright facing a large black X on a wall a few feet in front of them. They were told that they would hear loud sounds that might startle them. The sounds were 115-dB(A), 40-ms white noise bursts with 0-ms rise and fall times separated by between 18 and 22 seconds presented binaurally over 70-dB(A) background noise and generated by a San Diego Instruments Startle Reflex System (SR-Lab). Officers were instructed to keep their eyes open except when blinking, to focus their attention on the X and to listen to the sounds. Previous research found that physiologic responses to startling sounds were not only enhanced by an imminent electric shock but also to some extent by the mere placement of shock generating electrodes (e.g., Grillon & Davis, 1997). This suggested to us that we could create three progressively threatening contexts. Under the low threat condition, participants were told that they would receive shocks later in the study but that the shocks could not occur until they were fitted with a specific device (Coulbourn Instruments Transcutaneous Aversive Finger Stimulator Model E13-22). They were then presented with eight startling sounds. The first four responses were treated as habituation trials and the remaining responses were retained for analysis. Under the medium threat condition, participants were fitted with the finger stimulator but signaled with a colored light that they would definitely not be shocked. They were then presented with four startling sounds. Under the high threat condition, the officers continued to wear the finger stimulator but were signaled with a different colored light that they could receive up to three shocks at any time. They were then presented with four startling sounds. All participants received one 2.5-mA shock following the final sound of the high threat condition and all underwent the low threat condition first followed by the other two threat conditions in a counterbalanced order. The appropriate light signal remained on for the entire duration of the medium and high threat conditions, thus acting as a long lead stimulus of between 18 and 22 s (i.e., the length of the intertrial intervals). Response Measures Emotional Response Scale (ERS) The ERS was developed by the first author to assess self-reported emotional reactions to the startling sounds. Following each threat condition, officers rated how strongly they felt each of several emotions (e.g., anger, fear, amusement, calm) on a scale from 0 (not at all) to4 (quite a lot). Eyeblink Electromyogram EMG indexes changes in voltage associated with the strength of muscle contraction. EMG in microvolts was measured using three, 4-mm (sensor diameter) In Vivo Metrics Ag/AgCl surface electrodes filled with Signa Gel. Two electrodes were placed on the left orbicularis oculi according to published specifications (Fridlund & Cacioppo, 1986) and the ground electrode was placed behind the left ear. Impedance levels were kept below 10 k. The EMG signal was amplified, rectified, and filtered so as to retain the 10- to 500-Hz range, notch filtered at 60 Hz, sampled at 500-Hz, digitized for 4 s beginning with the stimulus onset, and stored for off-line analysis by the SR-Lab. Skin Conductance Level SC level indexes transient changes in the skin s ability to conduct electricity. It is associated with changes in

4 474 Pole, Neylan, Best, Orr, and Marmar sweat gland activity in response to arousing stimuli and primarily mediated by frontal, amygdala, and hypothalamic pathways via the sympathetic nervous system (e.g., Williams et al., 2001). SC level in microsiemens was obtained from the medial phalanges of the middle and index fingers of the left hand by using an Expanded Technologies skin conductance coupler with a constant 0.17 V through 2-mm (sensor diameter) Ag/AgCl electrodes filled with Signa Creme. Signals were amplified by a VI-MasterLab and sampled at 500 Hz, digitized for 4 s beginning with the stimulus onset, and stored for off-line analysis by the SR-Lab. PTSD Symptom Measure Mississippi Scale Police Version (MS-PV) The MS-PV is a 35-item self-report measure of cumulative PTSD-related symptoms adapted from The Mississippi Scale for Combat-Related Posttraumatic Stress Disorder (Keane, Caddell, & Taylor, 1988) by replacing references to the military with references to police service. Each item was rated on a 5-point scale from 1 (not at all true)to5(extremely true). The original Mississippi Scale has excellent psychometric properties (Watson, 1990). The MS-PV had comparable internal consistency (α =.93) and good concurrent validity as indicated by the strong correlation between the MS-PV total score and the CAPS total score, r(51) =.70, p <.001. Data Reduction Self-Reported Emotional Responses For each threat condition, we reduced the responses from the corresponding ERS to an index of reported danger emotions (Danger; the average of the ratings of fear, anxiety, worry, and danger ) and an index of reported safety emotions (Safety; the average of the ratings of calm, contentment, and safety ). The average internal consistency of the scales was α =.83 for Danger and α =.53 for Safety. Physiologic Responses The eyeblink EMG startle response (EMGR) occurs with short latency, usually within 200 ms of stimulus onset (Berg & Balaban, 1999). EMGR scores were determined by smoothing the raw EMG data with a 10-ms moving average and then obtaining the peak EMG value between 21 and 200 ms following stimulus onset subtracted from the mean EMG value during the first 20 ms. If no peak response was detectable (i.e., EMGR less than 1 µv) a zero response was scored. Responses were scored as missing (approximately 1% of the data) if EMG was unstable or if the peak EMG occurred within the first 20 ms. The SCR occurs more than 1 s following stimulus onset (Shalev et al., 1992). SCR was scored by subtracting the mean level during the first 500 ms following stimulus onset from the maximum SC level within 1 4 s poststimulus. We calculated two physiologic response scores (EMGR and SCR) for each threat condition by obtaining the mean response to the four stimulus presentations that comprised each condition. To determine whether four stimuli were sufficient to obtain a reliable estimate of reactivity to each condition, we calculated the internal consistency within each condition and found an average internal consistency of α =.94 for EMGR and α =.76 for SCR. PTSD Symptom Severity The MS-PV was reduced to a total score indexing the officers cumulative duty-related PTSD symptom severity by summing the responses to all 35 items. We also indexed self-reported exaggerated startle symptoms using the response to a single MS-PV item (no. 25): Since I became a police officer, unexpected noises make me jump. Five officers met the recommended Mississippi PTSD diagnosis cutoff score of 102 (Watson, 1990). We focused on PTSD symptom severity rather than on PTSD diagnosis because (a) few officers met full diagnostic criteria for PTSD; (b) the MS-PV scores were normally distributed enough to justify treating them as a continuous variable; and (c) subthreshold PTSD symptoms are so frequently observed among police officers that their effects, if any, deserve attention. Results Demographic Variables, PTSD Symptoms, and Their Interrelationship Of the 55 participants, 84% (n = 46) were male, 69% (n = 38) were married, 58% (n = 32) were European American, 16% (n = 9) were African American, 15% (n = 8) were Hispanic American, and 11% (n = 6) were Asian American. The officers, who were on average 34.8 (SD = 7.1) years old, had 15.1 (SD = 1.6) years of education and 11.2 (SD = 7.1) years of police service. We found no significant relationships between demographic variables and PTSD symptoms.

5 Startle, PTSD Symptoms, and Police 475 The Effects of Threat Condition on Self-Reported Emotions and Physiologic Responses Means and standard deviations of the response variables under each threat condition are shown in Table 1. We determined whether the threat manipulation led to changes in the response variables, using a repeated measures MANOVA in which threat condition (low, medium, and high) was treated as the within-participants variable and reported emotion (Danger and Safety) and physiologic (EMGR and SCR) responses were treated as simultaneous-dependent measures. We found a significant overall effect of threat condition, F(8, 46) = 17.35, p <.001, η 2 =.75. Univariate analyses (corrected with the Huynh Feldt epsilon when appropriate) revealed that the threat conditions produced significant effects on each dependent variable. Specifically, for the sample as a whole, the officers mounted larger EMGR under high threat than under medium threat, F(1, 53) = 11.68, p <.001, η 2 =.18, and larger EMGR under medium threat than under low threat, F(1, 53) = 22.80, p <.001, η 2 =.30. High threat produced larger SCR than did medium threat, F(1, 53) = 5.46, p <.05, η 2 =.09, but medium threat was not significantly more provocative of SCR than was low threat, F(1, 53) = 1.64, p >.05, η 2 =.03. In the realm of selfreported emotion, the officers reported the most Danger under high threat, less Danger under medium threat, F(1, 53) = 42.92, p <.001, η 2 =.45, and the least Danger under low threat, F(1, 53) = 5.29, p <.05, η 2 =.09. Finally, although officers reported the least Safety under the high, as compared to, medium threat, F(1, 53) = 14.62, p <.001, η 2 =.22; low threat did not elicit different reported Safety than did medium threat, F(1, 53) = 1.77, p >.05, η 2 =.03. The Relationship Between PTSD Symptoms and Laboratory-Based Startle Measures We next examined the relationship between PTSD symptoms and laboratory-based startle responses by conducting omnibus multiple correlation analyses within each threat condition entering all response variables simultaneously and relating them to the MS-PV score. We followed significant multiple correlations with an examination of the bivariate correlations between each response variable and the MS-PV score. Low Threat Condition The omnibus multiple correlation analysis revealed a significant association between PTSD symptoms and the response variables under low threat, R =.68, F(4, 50) = 10.93, p <.001, accounting for 47% of the variance in PTSD symptoms. An examination of the correlations with the individual response variables revealed that, under low threat, greater PTSD symptoms were significantly associated with larger SCR, r(53) =.54, p <.001, and greater reported Danger, r(53) =.46, p <.001. Medium Threat Condition The omnibus analysis also revealed a significant multiple correlation under medium threat, R =.61, F(4, 49) = 7.10, p <.001, accounting for 37% of the variance in PTSD symptoms. Bivariate correlations revealed that, under medium threat, greater PTSD symptoms were associated with larger EMGR, r(53) =.29, p <.05; larger SCR, r(53) =.48, p <.001; and greater reported Danger, r(53) =.38, p <.01. Table 1. Mean Physiologic and Reported Emotional Response to Startle Conditions Low threat Medium threat High threat Measure M SD M SD M SD Mean EMGR 36.8 bc ac ab 58.3 Mean SCR 0.20 c c ab 0.33 Danger 0.40 bc ac ab 0.83 Safety 1.26 c c ab 0.78 Note. Electromyogram response (EMGR) expressed in microvolts. Skin conductance response (SCR) expressed in microsiemens. Reported danger emotions (Danger) and reported safety emotions (Safety) expressed in subjective units, a 5-point Likert type scale (0 = none,1=a bit,2= some, 3=a lot, 4=quite a lot). Subscripts indicate a significant mean difference at p <.05 by the Fisher least significant difference test (a = significant difference from low threat condition, b = significant difference from medium threat condition, c = significant difference from high threat condition). High Threat Condition Finally, we found that the multiple correlation was also significant under high threat, R =.45, F(4, 50) = 3.18, p <.05, accounting for 20% of the variance in PTSD symptoms. Bivariate correlations revealed that, under high threat, PTSD symptoms were only related to greater reported Danger, r(53) =.36, p <.01. Did the Laboratory Measures Add to the Assessment of PTSD Symptoms? To address this question, we first examined the correlations between the self-reported emotion (Danger, Safety) and physiologic (EMGR and SCR) response measures and

6 476 Pole, Neylan, Best, Orr, and Marmar found no significant correlations between them. We next examined the relationship between reported exaggerated startle symptoms (MS-PV exaggerated startle item) and the laboratory response measures. We found that reported exaggerated startle symptoms were correlated with greater SCR under low threat, r(53) =.30, p <.05; greater reported Danger under both low threat, r(53) =.33, p <.05, and high threat, r(53) =.33, p <.05; and less reported Safety under high threat, r(53) =.27, p <.05. We then conducted hierarchical multiple regression analyses to determine whether the laboratory-based startle response measures improved upon reported exaggerated startle symptoms alone in assessing PTSD symptom severity, by entering the MS-PV exaggerated startle item in the first step, reported Danger and Safety emotions in the second step, and EMGR and SCR in the third step (Table 2). We found that the exaggerated startle item alone accounted for approximately 11% of the variance in PTSD symptom severity. Under low threat, the reported emotions added 14% to the explained variance and the physiologic measures added 22% more explained variance. Under medium threat, reported emotions added 13% to the explained PTSD symptom variance and the physiologic measures added 16% more. Under high threat, neither reported emotions nor physiologic responses added to the prediction of PTSD symptom severity above self-reported exaggerated startle symptoms. Another way to interpret this analysis is to examine which variables remain significant predictors of PTSD symptom severity when all predictors are entered into the model. In the full low and medium threat models, only reported Danger and SCR remained significant predictors of PTSD symptom level. In the full high threat model, only reported Danger was a significant predictor of PTSD symptom level (Table 2). Discussion Our major findings in this study were as follows: (a) lower levels of contextual threat were most effective in eliciting exaggerated responses to startling sounds in officers with high PTSD symptoms and (b) laboratory-based startle response measures obtained under these lower threat conditions added significantly to the assessment of PTSD symptom severity above and beyond what could be ascertained by a self-report measure of exaggerated startle symptoms alone. Our results were both consistent with previous studies (e.g., Grillon & Morgan, 1999; Lang et al., 1990; Metzger et al., 1999; Vrana, 1995) and novel in some respects. To our knowledge, this is the first study to examine the relationship between PTSD symptoms and physiologic startle responses in police officers, extending results observed in combat veterans (e.g., Morgan et al., 1996), sexual assault victims (Metzger et al., 1999), helpseeking civilian trauma survivors (Shalev et al., 1997), and other non-treatment-seeking participants (Orr et al., 1997). Our study is also unusual in the startle literature because we found that, in a potentially threatening situation, the relationship between PTSD symptoms and startle responding decreases as proximity to the threat increases. Assuming that the result is not simply an artifact of ceiling effects under high threat, it appears that all officers, regardless of PTSD symptoms, showed comparably large emotional and physiologic responses when confronted with the actual threat of imminent electric shock. However, under Table 2. Prediction of PTSD Symptom Severity Using Preexperiment Reported Startle and Laboratory Measures of Startle Under Three Threat Conditions Preexperiment Low threat a Medium threat b High threat c Variable B SE B β B SE B β B SE B β B SE B β Step 1 Reported exaggerated startle Step 2 Reported emotions Danger Safety Step 3 Physiologic response EMGR SCR Note. EMGR = Electromyogram response. SCR = Skin conductance response. The figures above refer to the full regression models. The changes in percentage of variance accounted for in each step and the significance of those changes are given below. a Step 1 (R 2 =.11, p <.05); Step 2 ( R 2 =.14, p <.01); Step 3 ( R 2 =.22, p <.001). b Step 1 (R 2 =.11, p <.05); Step 2 ( R 2 =.13, p <.05); Step 3 ( R 2 =.16, p <.01). c Step 1 (R 2 =.11, p <.05); Step 2 ( R 2 =.07, p >.05); Step 3 ( R 2 =.05, p >.05). p <.05. p <.01. p <.001.

7 Startle, PTSD Symptoms, and Police 477 low and medium threat, officers with greater PTSD symptoms reacted as if they were under high threat, perhaps out of an inability to inhibit worry about the shock that was to come later in the experiment. Grillon and Morgan (1999) reported something similar, namely that veterans with PTSD showed elevated startle in the presence of both safety and danger cues. However, our finding in police officers may have special significance because of the nature of police work. Urban police officers are, in the course of their regular duties, often in the position of judging the safety or dangerousness of a situation and reacting accordingly. Errors of judgment in either direction can have deadly consequences. Our future studies will examine whether these laboratory measures are associated with measures of job performance including accuracy of risk appraisal. An examination of which specific measures were correlated with PTSD symptoms and under which conditions is also illuminating. For example, regardless of threat condition, severe PTSD symptoms were always associated with greater reported Danger emotions but never significantly related to lower reported Safety emotions. This suggests that although highly symptomatic officers were feeling greater Danger throughout the experiment, they were simultaneously reporting feeling about as Safe as their nonsymptomatic counterparts. Thus, despite the abnormality in their experience of danger-like negative emotions, their capacity to feel safety-like positive emotions was relatively intact. The failure to find PTSD-related effects for the Safety emotions could, however, be due to the relatively weak internal consistency of the Safety scales. Turning to the physiologic measures, we found that PTSD symptoms were related to SCR under both low and medium threat conditions but were only significantly related to EMGR under the medium threat condition. This finding is consistent with both reports of a robust PTSD/SCR relationship in the absence of significant threat (e.g., Shalev et al., 1992, 1997) and reports of a robust PTSD/EMGR relationship only with the introduction of contextual threat (Grillon et al., 1998a, 1998b; Morgan et al., 1995). To our knowledge, until now, no study has examined the link between PTSD and both EMG and SC responses in a fear-potentiated startle paradigm. Our results highlight the differences between these two physiologic measures and might be interpreted as indicating that PTSD symptoms covary with abnormalities in both general defensive responding (i.e., exaggerated SC responding) and the startle reflex (i.e., exaggerated eyeblink EMG responding) depending upon contextual threat. Finally, although both the emotional and the physiologic startle measures were related to overall PTSD symptom severity, they were not related to each other. A simple interpretation of this result is that the two types of laboratory measures tap nonoverlapping aspects of PTSD symptom severity. Yet, it may also suggest that there are two types of officers with high PTSD symptoms. One type reports larger emotional responses to the laboratory manipulation whereas another type exhibits greater physiologic responses. Our future research will investigate whether exaggerated physiologic responses in the latter subtype are a consequence of suppressing their emotions. Active suppression of emotions has been found by some investigators to increase physiologic reactivity (Gross & Levenson, 1997). The study has several limitations that restrict the conclusions that may be drawn. For example, although we were able to counterbalance the medium and high threat conditions, the low threat condition was always administered first raising concerns about habituation or sensitization of startle responding. We are somewhat reassured that prior exposure to the low threat condition did not eliminate the potency of the other conditions because we found that, when the sample was examined as a whole, the high threat condition, as compared to the low threat condition, produced less safety emotions, more danger emotions, and larger physiologic responses. Also, even though the order of the medium and high threat conditions was varied, the dependent measures generally preserved the expected effects of increasing threat. Still, future studies might consider attempting to counterbalance low threat by removing the shock electrodes at counterbalanced times throughout the study. A second potential limitation is that we had few actual PTSD cases in our sample. Previous research in this area has tended to examine differences between groups of participants with and without PTSD. Although our study was different in this respect, by using a multiple regression approach to analyze our data, we were able to highlight the linear association between PTSD symptom severity and startle responding, an analysis often missing in group comparison studies. In addition, we believe that for nontreatment-seeking active duty police officers, the effects of subsyndromal PTSD symptoms are relevant because of the level of functioning required to perform their duties and the duty-related risk for further traumatization. Clearly, understanding the effects of these subclinical symptoms would be of interest to police departments throughout the country. Limitations notwithstanding, our results show that the experimental context plays a major role in differentiating police officers with lower versus higher levels of PTSD symptoms. Officers with high levels of PTSD symptoms appear to attribute higher levels of threat to low and medium threat contexts. This provides biological support for the view that police officers with greater PTSD

8 478 Pole, Neylan, Best, Orr, and Marmar symptoms subjectively view the world as more dangerous, uncontrollable, and unpredictable than objectively warranted. Finally, our results show that the time and expense involved in collecting laboratory measures of startle can yield an incremental gain in the ability to assess PTSD symptom severity beyond what might be afforded by simply obtaining the officer s report of exaggerated startle symptoms. Having laboratory methods to assess PTSD in police may be particularly pertinent because of potential institutional pressures to deny or underreport emotional distress. Thus, this approach may offer new possibilities in the assessment of PTSD in law enforcement professionals. Acknowledgments This project was supported by NIMH Minority Supplement (R01 MH ) to the first author to investigate acoustic startle in police officers. The authors thank Malcolm Gordon for his assistance with the supplement application; Joyce Sprock, Marja Germans, Andy Morgan, Christian Grillon, Jeffrey Hille, and Charles Bub for their expert consultation; Thomas Metzler for his data management; David Mohr for his comments on the manuscript, and Delores Carter Pole for her special contributions. Finally, we express our appreciation to all of the police officers who participated in this research. References American Psychiatric Association. (1994). Diagnostic and statistical manual of mental disorders (4th ed.). Washington, DC: Author. Berg, W. K., & Balaban, M. T. (1999). Startle elicitation: Stimulus parameters, recording techniques, and quantification. In M. E. Dawson, A. M. Schell, & A. H. Bohmelt (Eds.), Startle modification: implications for neuroscience, cognitive science, and clinical science (pp ). New York: Cambridge University Press. Blake, D. D., Weathers, F. W., Nagy, L. M., Kaloupek, D. G., Charney, D. S., & Keane, T. 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